DS80C390_00 Datasheet, PDF (151/155 Pages) Dallas Semiconductor – High-Speed Microcontroller User’s Guide Supplement

DS80C390 High-Speed Microcontroller User’s Guide Supplement
nominal bit time = t_{SYNC_SEG} + t_TSEG1 + t_TSEG2
= ^{(BRPV)(CCD)[1 + (TS1_ LEN) + (TS2 _ LEN)]}
FOSC
maximum bit time = t_{SYNC_SEG} + t_TSEG1 + t_TSEG2 + t_SJW
(BRPV)(CCD)[1 + (TS1_ LEN) + (TS2 _ LEN) + (SJW)]
=
FOSC
CAN baud rate
=
FOSC
(BRPV)(CCD)[1+ (TS1_ LEN) + (TS2 _ LEN)]
Threefold Bit Sampling:
The DS80C390 supports the ability perform one or three samplings of each bit, based on the SMP bit
(CxBT1.7). The single sample mode (SMP=0) is available in all settings and takes one sample during
each bit time. The triple sampling mode (SMP=1) samples each bit three times for increased noise
immunity. This mode can only be used when the baud rate prescale value (BPRV) is greater than 3.
Bus Rate Timing Example:
The following table shows a few example bit timing settings for common oscillator frequency and baud
rate selections. Because of the large number of variables, there are many combinations not shown that can
achieve a desired baud rate. There are a number of approaches to determining all the bit timing factors,
but this utilizes the most common, i.e., the oscillator frequency and baud rate have already been
determined by system constraints.
Additional Bit Timing Examples (assumes CCD=1)
F_OSC Baud rate BRPV CCD t_QU t_QU TS1_LEN TS2_LEN
per bit
40 MHz 1 Mbps 2
2 100 ns 10
5
4
500 kbps 4
2 200 ns 10
5
4
250 kbps 5
2 250 ns 16
10
5
125 kbps 10
2 500 ns 16
10
5
SJW
3
3
4
4
SMP=1
Permitted?
NO
YES
YES
YES
16 MHz 1 Mbps 1
2 125 ns 8
4
500 kbps 1
2 125 ns 16
10
250 kbps 2
2 250 ns 16
10
125 kbps 4
2 500 ns 16
10
3
4
NO
5
4
NO
5
4
NO
5
4
YES
8 MHz 1 Mbps 1
1 125 ns 8
4
500 kbps 1
1 125 ns 16
10
250 kbps 1
1 250 ns 16
10
125 kbps 2
2 500 ns 16
10
3
2
NO
5
4
NO
5
4
NO
5
4
NO
As an aid to understanding, the following is an explanation of how the table row illustrating an oscillator
frequency of 16 MHz and a CAN baud rate of 125 kbps is derived.
Various combinations of BRPV are selected until one is located that meets the "t_QU per bit" criteria, i.e.,
an integer value less than 24. Selecting BRPV=4, the previously described equations state that there
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